Current external refining methods for industrial scale production of ultra-low carbon, ultra-low sulfur steel mainly adopt a RH vacuum refining. The main problems of the process for producing the ultra-low carbon, ultra-low sulfur steel by using the RH vacuum refining are following:
First, a decarburization velocity of the RH vacuum refining is low, because in case of the same molten steel active oxygen, carbon content and vacuum degree, the reaction velocity of the vacuum decarburization mainly depends on a circulating flow rate of a molten steel, and there is a 1.5 order relationship between the molten steel circulating flow rate and a inner diameter of a snorkel. Because a RH furnace consists of two upward and downward snorkels, the inner diameter of the snorkel is smaller, the decarburization velocity is restricted by an inner diameter of the snorkel and it is difficult to be improved by a process optimization.
Second, a desulfurization rate of the RH furnace vacuum refining process is low and unstable, and a desulfurizer severely corrodes the snorkel and a refractory material of a lower part slot in a vacuum chamber. The reason is the desulfurization in the RH furnace vacuum process is primarily to disperse the desulfurizer within the molten steel by using a circulating motion of the added desulfurizer with the molten steel, forming a reaction interface to achieve the purpose of the desulfurization. This requires that 1) the addition of the desulfurizer is primarily by mixing and injecting a gas and powder two-phase steam into a molten pool of the vacuum chamber, to facilitate dispersion of the desulfurizer within the molten steel; 2) the desulfurizer must be a mixture with high sulfur melting quantity and low melting point (it is the commonly used mixture containing 30% CaF2 and 70% CaO); 3) a circulating flow rate of the molten steel is sufficiently high, to ensure more molten steel entering into the vacuum chamber and contacting with the desulfurizer particles over the same powder injection time, improving the desulfurization efficiency. In view of this, the desulfurization in the RH vacuum process has the following inherent defects: 1) because a desulfurization rate depends on a dispersion degree of the desulfurizer within the molten steel, the desulfurization rate is unstable; 2) the desulfurization rate is highly effected by the circulating flow rate of the molten steel, the circulating flow rate of the molten steel under the two snorkel conditions of RH is small, thus the desulfurization rate is not high; 3) because the desulfurizer contains up to 30% of CaF2, it severely corrodes the vacuum chamber and the refractory material of the snorkel, shortening its service life.
To date what have been disclosed in China are Chinese Patent Number ZL00235854.9 “Multi-function combined blown single nozzle refining furnace”, Chinese Patent CN101701279B “A method for smelting low-sulfur steel by a single nozzle refining furnace” and Chinese Patent CN101792845B “A method for smelting ultra-low carbon steel by a single nozzle refining furnace”, these patents improve the circulating flow rate of the molten steel in the vacuum refining process by increasing a cross-sectional area of the snorkel, so as to improve a decarburization efficiency. However, in order to ensure that an exposed face of the molten steel in the vacuum chamber being sufficiently large to improve a decarburization reaction velocity, these patents all require discharging a steel ladle slag from the snorkel before the snorkel is inserted into the molten steel, increasing the exposed face of the molten steel, and improving the reaction velocity of the decarburization. Meanwhile, the desulfurizer injected during the desulfurization can effectively disperse within the molten steel, thereby increasing the reaction interface and improving the desulfurization efficiency. Accordingly, these patents have strict requirement on a tapping slag quantity in a converter (or electric furnace) tapping process. If a surface slag layer of the molten steel is thick, then a slag discharge operation before insertion of the snorkel into the molten steel is difficult to conduct. These patents are same as the RH vacuum refining in terms of desulfurization principle of the vacuum process, thus it is necessary to select a desulfurizer with high CaF2 content (generally 30%) which is same as the sulfurization in the RH vacuum refining process, the desulfurizer severely corrodes the snorkel and the refractory material of the lower part slot in the vacuum chamber, decreasing the service life of the snorkel and the lower part slot in the vacuum chamber. The single nozzle refining furnaces which have been disclosed to date all use steel ladle bottom blowing as a driving force for a circulating flow of the molten steel in the vacuum process, and when a clogging occurs in the gas permeable brick at the bottom of the steel ladle, the vacuum refining will be unable to conduct, resulting in production interruption.
Chinese Patent CN101302571A discloses a single nozzle refining furnace, at least of a set of traveling magnetic field generators disposed on a periphery of its suction nozzle is merely used for increase the flow velocity of the molten steel, improving the circulating flow rate of the molten steel. It is unable to resolve the problem of steel ladle top slag layer covering the molten steel face and resulting in decreased expose face of the molten steel, and decreasing the decarburization and desulfurization efficiency. Also, in Chinese Patent CN101302571A, the traveling magnetic field generators on the snorkel can accelerate the flow velocity of the molten steel only at the time of circulating flow, once the steel ladle bottom blowing is clogged and the molten steel is in motionless state, the traveling magnetic field generators will be out of action, and the vacuum refining is unable to be conducted, resulting in production interruption.